Method for increasing production of hydrocarbon bearing wells by treatment with hot acid solutions



Ft nti 3,367,417 METHOD FOR INCREASING PRODUCTION OF HY DROCARBON BEARING WELLS BY TREAT- MENT WITH HOT ACID SOLUTIONS Edward D. McCabe, Pampa, Tex., assignor to Halliburton Company, Duncan, Okla, a corporation of Delaware Filed Dec. 17, 1965, Ser. No. 514,715 9 Claims. (Cl. 166-4) ABSTRACT OF THE DISCLOSURE This invention relates to a method for treating a well in a hydrocarbon bearing formation to increase it productivity. It involves heating an aqueous well treating fluid, subsequently adding a low molecu ar weight carboxylic acid such as acetic acid, and then introducing this solution into a well formation.

This invention relates to a method of treating wells to improve their productivity. More specifically, the invention relates to a method of treating wells within an aqueous acid medium and preferably one containing a low molecular weight carboxylic acid. It is a frequent experience that the productivities of many hydrocarbon bearing formations may become greatly decreased. There are many reasons for this decreased productivity, for example, if the formation contains an oil of low or medium viscosity at reservoir conditions, oil flow may be simply prevented by the high viscosity. Another source of decreased productivity is a reduction of permeability especially in the area surrounding the bore hole. A possible cause of the reduced permeability is swelling and caking of clays present in the porous medium when contacted by fresh water or drilling mud filtrates. Thus, the swelling of the clay may greatly reduce the porosity of the formation and result in a decreased productivity.

It is known that the productivity of a well may be increased by flooding of the underground formation and especially when the flooding is carried out with hot water or with steam. However, since the decreased productivity may be caused at least in part by the contact of water with various clays, an aqueous treatment alone may be ineffective or actually result in a further reduction in permeability. The tendency of the cay to swell upon treatment with water can be reduced somewhat by using saline water. Unfortunately, the water usually available near the formation contains alkaline earth metal salts such as those of calcium and magnesium. These salts frequently tend to cause scaling in the well formation and thus are somewhat unacceptable.

In a typical method of treating a well with a heated aqueous fluid, water is first heated to the desired treating temperature. The heating step may be such as to convert the water to steam or merely heat the water to high temperature while it remains in the liquid state. The heater may be any suitable means of raising the temperature of Water as, for example, a shell and tube heat exchanger or a heat exchanger in the form of a spiral of coils through which water flows with a suitable burner positioned at the bottom to heat the coils. Steam or high temperature water produced in the heating unit is then cartied down the well through tubing to be discharged at the bottom preferably in the zone adjacent to the production formation. The water entering the heater is likely to be quite basic having a pH of say about 10. This high pH is desirable because it tends to prevent corrosion in the heater. However, it may be undesirable to introduce water containing such a high pH and thus such a high salt concentration into the formation because of scaling problems.

"ice

Summary The present invention provides a method for treating hydrocarbon bearing wells whereby production of the well may be increased as much as five times under conditions where steam treatment alone would be ineffective in increasing production. The method provides heat treating and acid treatment but at the same time results in little if any corrosion in the heating equipment and little corrosion on metal in the wel'. According to the invention, the aqueous treating fluid is heated and then a low molecular weight carboxylic acid is added at the outlet side of the heater to reduce the pH to a range of about 4 to 6. The resultant solution is introduced into a hydrocarbon bearing well. Corrosion by the acid solution is effectively avoided since the acid solution can be effectively inhibited to prevent metallic corrosion and since the acid is added after the heater thereby avoiding acid corrosion problems. Continuous automatic acid addition may be provided by the use of a pH analyzer connected to the acid metering device.

It is thus a primary object of the present invention to provide a method otfloodingamunder l lld ilP ing formation wtih an aqueous fluid that has no tendency to plug the underground formation into which it is being injected.

It is a further object of the present invention to provide a method for decreasing the plugging tendency and high pH of an aqueous treating fluid for formations containing water sensitive materials.

It has now been foundthat the method of treating a well in a water sensitive formation with an aqueous fluid can be greatly improved by using a fluid containing an acid such as a low molecular Weight carboxylic acid like acetic acid. The use of an acid in the aqueous well treat- 5 ing fluid will result in a much greater productivity than could be obtained by using the aqueous fluid alone. Although the exact nature of the interaction of the acid or acid forming material with the well formation to result in increased well capacities is not known, there are several explanations for this unexpected improvement obtained. Addition of the acid to the aqueous treating fluid will decrease the pH and tend to inhibit precipitation of various salts which could cause scaling. Furthermore, the acid will chemically react on clay pa rticles to tocmhydr g n c y..iu, h.is..s t W t acid .envirqnment, the clay particles will remain in the least swollen condition.

In general, both inorganic and organic acids including acid producing chemicals may be used in this invention. The preferred acids are carboxylic acids containing from about 1-6 carbon atoms, e.g., formic, acetic, propionic, butyric, valeric, and caproic. Other organic acids which may be used are, for example, benzoic acid, phthalic acid, and salicylic acid. The preferred acid is acetic acid which exhibits a great effect in increasing permeability, is a relatively strong acid, prevents the formation of scale, and yet is relatively nonco-rrosive with respect to the metal parts in the well.

Inorganic acids which may be used include phosphoric acid, hydrochloric acid, sulfuric acid, nitric acid, etc. Also within the scope of the invention are materials which will form an acid when added to water such as aluminum chloride, ammonium chloride, ferric chloride and cupric chloride.

As previously indicated, one of the desirable characteristics of acetic acid is that it is relatively noncorrosive. Although stronger acids are quite suitable for use in the present invention. It is preferable to use them in conjunction with a corrosion inhibitor. The use of the strong acid alone would be likely to cause great damage to the well equipment through corrosion. In selecting a corrosion inhibitor, the high temperatures used in the process should be kept in mind and an inhibitor which will not break down at these temperatures should be used.

Although the effective amount of acid can vary over a irelatively wide range, the preferred method of measuring {the amount of acid present in the well treating fluid is l to add enough acid so that the pH of the aqueous treating fluid is adjusted to between about 4 and 6.5 and preferably ibetween about and 5.5. It has been found when the acid is added to obtain this predetermined pH level, that the most effective increase in well productivity can be obtained.

It may also be desirable to add special surfactants to the organic acid in order to make the acid compatible with crude oils and thus minimize emulsion formation within the well. The surface-active materials may be added with the acid prior to addition of acid to the aqueous fluid or aftgg addition of acid.

/---In a typical well treating operation according to the present invention, raw water is first introduced into a heater where it is heated to a temperature high above its normal boiling point under pressure, say about 400600 F. at a temperature sufficient to keep it in the liquid state. During this heating operation, it is desirable to maintain the pH of the water as high as possible in order to reduce corrosion problems, and thus it is usually not desirable to add the acid prior to this point. The s t e ai n g r sp L- heated water isthen discharged into th e well formation by means of suitable pipes and valves, etc. This operation conventional and various modifications are well-known to those skilled in the art.

After heating the aqueous fluid, and before discharging it to the well, the acid is added Any suitable means of adding the acid may be used and, for example, the acid may even be poured directly down the bore hole at the time that steam is entering the bore hole. However, a much more suitable method of adding the acid is to meter it into the pipe carrying steam or superheated water from the heater to the bore hole. The metering can be carried out by any metering-pump which will pump the acid into the steam line. It will, of course, be necessary to carry out routine tests to determine the amount of acid required to lower the pH of the steam or water from the pH of the crude water to that in the range of about 4-6. This can easily be done by simply taking samples of the water at a point far enough downstream from the acid addition so that it has been thoroughly mixed and conducting routine analyses to determine the pH. Acid addition may be continuous or incremental. Continuous, automatic addition may be provided for by the use of a pH analyzer connected to the acid metering device.

The method of this invention can be more fully understood by reference to the following examples.

Example 1 A well in Gray County, Tex., near Pampa, was treated as follows. To the raw water used in the process, ten pounds of sodium sulfide and five pounds of sodium hydroxide were added in each one hundred barrels of water. The water thus treated had a pH of 10, an oxygen content of less than 1, and less than 1 part of hardness. The water was then introduced into a propane burning water heater and heated to a temperature of from about 450-490 at a pressure of from about 425 p.s.i. to 550 p.s.i. over a period of several days. The average pumping rate was about 22 gallons per hour. On the outlet side of the heater, acetic acid was metered into the water line by means of a chemical pump to obtain a pH of between 5 and 5 /2. Approximately 20 gallons of acetic acid per day were required to maintain the desired pH. After several days of treatment, it was found that the productivity of the well had greatly increased.

Example 2 A well in Gray County, Tex., in Granite Wash formation, which is considered to be a large granular, highlypermeable formation having some clays therein, was tested. The well was treated with steam containing enough acetic acid to adjust the pH to 5. The steam treatment continued for 120 hours and at the end of that time, the capacity of the well had increased roughly 5 times.

Example 3 A second well in Gray County, Tex., in the Granite Wash formation, was treated with steam containing no acetic acid for hours. After the end of the steam treatment, the capacity of the well showed no increase.

I claim:

1. A method of treating a well in a hydrocarbon hearing formation for increasing the productivity thereof comprising passing an aqueous well treating fluid with a pH of at least 7 into a heating means, removing said fluid from the heating means, adding a low molecular weight carboxylic acid to said fluid in an amount sufficient to reduce the pH of the treating fluid to a range of from 4 to 6, and introducing the resultant solution into said formation.

2. The method of claim 1 wherein the acid is acetic acid.

3. The method of claim 2 including analyzing the pH of the treating fluid and continuously adding the acid at a rate determined by said analysis.

4. The method of claim 1 wherein the acid is formic acid.

5. The method of claim 1 wherein the heated well treating fluid is hot water.

6. The method of claim 1 wherein the heated well treating fluid is steam.

7. The method of claim 6 wherein a sufficient amount of acid is added to lower the pH to between about 5 and 5.5.

8. The method of claim 7 wherein the acid is formic acid.

9. The method of claim 7 wherein the acid is acetic acid.

References Cited UNITED STATES PATENTS 1,999,146 4/1935 Ambrose et a1 166-42 2,045,899 6/ 1936 Davis 252-855 2,265,923 12/1941 Normand 166-40 2,850,098 9/1958 Moll et al 166-42 3,122,204 2/1964 Oakes 166-42 3,237,692 3/1966 Wallace et al 166-40 3,249,536 5/1966 Jones 166-42 3,251,415 5/1966 Bombadieri et al. 166-42 OTHER REFERENCES Kingston, Benson M.: Acidizing Handbook, Houston, Tex.; Gulf Publication Co. 1936, pp. 20 and 30-32.

ERNEST R. PURSER, Primary Examiner.

IAN A. CALVERT, Assistant Examiner. 

